1. Field of the Invention
This invention relates to an X-ray microscope and particularly to an imaging X-ray microscope using a Schwarzschild optical system as its objective lens and utilizing the wavelength in the range of soft X-rays.
2. Description of the Related Art
Recently there has been a strong demand for observing an object image with high resolution using X-rays of a wavelength shorter than that of visible light, and X-ray microscopes have been developed in response to that demand.
Two types of X-ray microscopes are known: the scanning type and the imaging type. As shown in FIG. 1, a scanning X-ray microscope comprises an X-ray radiation source 1, a pin hole 2, an objective lens 3, a specimen 4 arranged movably in directions perpendicular to the optical axis of the objective lens 3, and an X-ray detector 5, all of which are arranged on the common optical axis. X-rays passing through the pin hole 2 are focused as a minute light spot on the specimen 4 by the objective lens 3, and the specimen 4 is moved in a plane perpendicular to the optical axis whereby a predetermined region of the specimen 4 is scanned to detect an image of the specimen having a certain size.
On the other hand, as shown in FIG. 2, an imaging X-ray microscope has a structure in which an X-ray radiation source 1, a condenser lens 6, a specimen 4, an objective lens 3, and an X-ray detector 5 are arranged coaxially. X-rays from the X-ray source 1 are focused on a region of a predetermined area on the specimen 4 by the condenser lens 6. The X-rays transmitted through or diffracted by the specimen 4 are focused on the detector 5 by the objective lens 3, and an image of the object having the predetermined size is formed.
As an optical system to be used as the objective lens of such an X-ray microscope, the Schwarzschild optical system is known. As shown in FIG. 3, this optical system comprises a concave mirror 7 having an opening in its center, and a convex mirror 8 which is arranged to oppose to the opening of the concave mirror 7. Light from the object point 0 is reflected successively by the concave mirror 7 and the convex mirror 8 to form an object image at the image point I.
When an imaging X-ray microscope is designed by using this Schwarzschild optical system as its objective lens, it is necessary to form an object image of a relatively large image height, thus the aberrations of the objective lens including offaxial aberration should be corrected well. Further, in order to obtain an image of sufficient brightness and high resolution, the numerical aperture on the object side of the objective lens must be large. Moreover, it is also necessary to prevent the deterioration of the imaging performance due to the error of assembly adjustment of the optical system.
There are two types of Schwarzschild optical systems: the concentric optical system in which the center of curvature C.sub.1 of the concave mirror 7 is identical with that of curvature C.sub.2 of the convex mirror 8, and the heterocentric optical system in which the center of curvature C.sub.1 of the concave mirror 7 is not identical with that of curvature C.sub.2 of the convex mirror 8. When viewed as the objective lens of an imaging X-ray microscope, these types have the following characteristics:
An example of the concentric Schwarzschild optical system is disclosed by P. Erdoes, Opt. Soc. America 49, 877(1959). In such an optical system, a strict degree of precision is required in its assembly adjustment and its error influences the imaging performance greatly. This will explained below.
FIGS. 4 and 5 are the diagrams for explaining the relationship between the concave mirror 7 and the convex mirror 8, and FIG. 6 is an enlarged view of the center of curvature in FIG. 4. In the figures, C.sub.1 and C.sub.1 ' are the centers of curvature of the concave mirror 7, C.sub.2 is the center of curvature of the convex mirror 8, d and d' are the distances between the centers of curvature of the concave mirror 7 and the convex mirror 8, and Z and Z' are the optical axes of the Schwarzschild optical system.
As shown in FIGS. 4 and 6, assume that the concave mirror 7 (having a radius of curvature r.sub.1) becomes eccentric and its center of curvature shifts from C.sub.1 to C.sub.1 ', that is, the concave mirror 7 rotates counterclockwise by an angle .theta. around the point of intersection of the optical axis Z and the concave mirror 7. Then the optical axis shifts from the straight line Z passing through C.sub.1 and C.sub.2 to the straight line Z' passing through C.sub.1 ' and C.sub.2. The difference between the distance d from C.sub.1 to C.sub.2 and the distance d' from C.sub.1 ' to C.sub.2 indicates the influence of eccentricity. Using the eccentric angle 0, d'-d is represented as follows: ##EQU1##
Further, as shown in FIG. 5, assume that the concave mirror 7 is displaced in a direction perpendicular to the optical axis Z and the center of curvature shifts from C.sub.1 to C.sub.1 '. If the distance between C.sub.1 and C.sub.1 ' is indicated by .DELTA.v, the difference between the distance d' from C.sub.1 ' to C.sub.1 and the distance d from C.sub.1 to C.sub.2 is represented as follows: ##EQU2##
As is apparent from equations (1) and (2), the influence of eccentricity is proportional to 1/d. Thus, a concentric Schwarzschild optical system in which d is zero or nearly equal to zero has the problem that the deterioration of performance due to the eccentric error is substantial. Therefore, the heterocentric optical system is advantageous in view of the eccentric error.
Hence the heterocentric optical system will be discussed below. As a measure of the deviation of the centers of curvature of the concave and convex mirrors of the Schwarzschild optical system, the heterocentric quantity DC defined by the following is introduced: ##EQU3## As examples of the heterocentric Schwarzschild optical system, I. Lovas, High Resolution Soft X-ray Optics, SPIE vol. 316(1981) discloses an optical system having DC .perspectiveto.-0.022 to -0.071 and the object-side numerical aperture NA =0.2, and SPIE vol. 563(1985) discloses an optical system having DC.perspectiveto.-0.06 and the object-side numerical aperture NA=0.2, 0.3 and 0.4.
However, the former optical system cannot provide sufficient image brightness since its numerical aperture is small. The latter optical system is difficult to use as the objective lens for the imaging X-ray microscope since offaxial aberration is large.
On the other hand, with respect to the aberration correction of the Schwarzschild optical system, Japanese Patent Publication No. 29-6775 is known. This discloses a method of determining the respective design parameters of the Schwarzschild optical system with the aberration correction considered, whether it is of the concentric or heterocentric type. The optical system analyzed there is designed for infinity, that is, the axial light beam exiting from the Schwarzschild optical system is parallel to the optical axis. As shown in FIG. 7, the state of correction of spherical aberration S and coma F is analyzed by representing the ratio r.sub.2 /r.sub.1 (=a) of the radius of curvature r.sub.2 of the convex mirror 8 to the radius of curvature r.sub.1 of the concave mirror 7 along the horizontal axis and the ratio d/r.sub.2 (=b) of the distance d between the centers of curvature of both mirrors to r.sub.2 along the vertical axis. It is disclosed that if the optical system is designed in the range of the hatching in the figure, that is, 3.ltoreq.1/a.ltoreq. 14, -0.5.ltoreq.S.ltoreq.0.2, b.gtoreq.0, then spherical aberration can be kept small. It is also disclosed that remaining aberration can be corrected well by coating the reflecting surfaces of the optical system designed as described above with a proper material to form aspherical surfaces.
However, if the Schwarzschild optical system is designed to satisfy the condition b.gtoreq.0 given there, the rays may be eclipsed at the edge of the convex mirror. Further, considering the simplicity of production of the reflecting mirror, it is not practical to make the mirror surface aspherical.